Prior art cryogenic refrigerators provided heretofore and capable of providing refrigeration in the range of 4.5.degree. to 10.degree. K. are limited to applications of technical endeavor where cost is not an overriding consideration and highly technical staffs are available to maintain the refrigerators. A simple system using liquid helium from a bulk liquid helium storage dewar, which is transferred to a device for test or operation, requires personnel with a large amount of technical skill just to accomplish the transfer with a minimum of losses.
The cost of liquid helium alone makes such a system impractical for continued research and/or operation. Such systems are usually limited to a few tests necessary to provide operating characteristics or proof-of-principle. For long term operation, most users prefer closed cycle refrigerators. The present equipment, used extensively in the area of radio telescopes, makes use of a two-stage cryogenic refrigerator to cool a stream of high pressure helium to a low enough temperature so that a net refrigeration effect is realized when the helium is expanded through a Joule-Thomson valve. That system is reasonably efficient but complicated and costly while requiring trained personnel for operation and maintenance. In addition, the Joule-Thomson valve is extremely sensitive to impurities in the helium because the orifice area of the valve is typically 3.6.times.10.sup.-2 mm.sup.2 which gradually becomes restricted, further causing a deterioration in a refrigeration capacity.
The problem addressed herein is how to develop a small closed cycle cryocooler producing refrigeration in the temperature range of 4.5.degree.-10.degree. K. Most of the work done by others for improving a prior art refrigerator relates to the regenerator matrix and in particular the design and materials thereof. Since helium is the working fluid, it is obvious that there is a mismatch between the heat capacities of the regenerator matrix and helium. In order to balance the two, it is necessary to increase the amount of matrix which increases the void volume of the regenerator and/or the pressure drop. In order to attain the above-mentioned temperature range, it appears necessary to have very large regenerator masses such as 6,000 grams and heat transfer areas such as 20,000 cm.sup.2. These heat exchangers are considered to be too large to be of any practical value. When the regenerator mass is reduced down to a realistic value such as 500 grams, approximately twice the mass used in standard machines today, cooling and/or net refrigeration decreased below 10.degree. K. with apparent warming occurring in the region of interest.